Vitamin is an umbrella term for a number of unrelated organic substances necessary in trace amounts for normal metabolic functioning of the body. Vitamins are involved in many biochemical reactions inside of us. When sufficient amounts of a particular vitamin are lacking in a persons diet, the processes that require them will either proceed incorrectly, or not at all. When this balance is upset, disease develops.

Vitamins are found in many types of food. Different foods also contain a wide variation in the percentages of each vitamin present. Some foods may contain insufficient amounts (if any) of one vitamin, while containing an abundance of another particular vitamin. This is the premise of a proper diet (combining different foods to ensure the necessary amounts of each nutrient. Vitamins are also produced by our bacterial flora residing in the gut. Bacteria occupying our gut aid digestion by breaking down undigested food. They also form a protective barrier, attempting to prevent more virulent bacteria, and other microbes from establishing residency in our bodies. Through metabolism during food breakdown, different species of bacteria may produce small amounts of vitamin K, niacin, riboflavin, vitamin B₁₂, and folic acid. However, the amounts of these vitamins produced by bacterial flora in the gut are usually insufficient and exogenous supply is required through food and vitamin supplements.

A minimum amount of each vitamin is needed in order for the biochemical systems of the body to function both correctly and efficiently. To simplify dose of each vitamin, we have come up with the term, recommended daily allowance (RDA). The recommended daily allowance of a vitamin, is an amount of a particular vitamin needed on a daily basis to function normally. We must not forget that each RDA for a particular vitamin is based on averages in the population. These are not always correct for each individual, but can be tailored accordingly. The RDA is only a guideline, and with good intentions. The RDA keeps things simple, assuming that we are not all well-versed in the sciences behind more accurate calculation of need. But as we learn, we shall see that RDA can vary considerably depending on body weight, sex (male, female), metabolism, lifestyle, genetics, and disease states. Strong variations in these variables can cause deficiencies leading to disease as will cover.

Each vitamin has a particular function in the body. Some vitamins perform functions such as helping us to obtain energy from the sugars we consume. Other vitamins help to properly build connective tissues. Some vitamins act as cofactors, binding to enzymes and hormones, enabling them to work more efficiently (a catalyst, in a sense). Other vitamins work as intermediaries in reactions during metabolism. We shall discuss each vitamin in detail as we move along. The important thing to keep in mind as we continue is that each vitamin is essential to a particular reaction or set of reactions in the body. As you will discover, every vitamin has a "vital" role in our health. In fact, the word vitamin was coined from scientists realising that these substances had a "vital force" necessary for life.

Vitamin classification

We can classify vitamins into two main categories, 1)fat-soluble, and 2)water-soluble. Being soluble simply means having the ability to dissolve in. Therefore, fat-soluble vitamins can dissolve in fat, and water-soluble vitamins have the ability to dissolve in water. By dissolving in water or fat, vitamins can then be absorbed by, transported into, and utilised by the body.

The fat-soluble vitamins include vitamins A, D, E, and K. The fat-soluble vitamins tend to be stored in the body in moderate amounts compared to water-soluble vitamins, and they are not normally excreted in the urine.

Water-soluble vitamins are all the vitamins that are soluble in water (ie., all but A, D, E, and K, which are fat-soluble). Basically, the water-soluble vitamin group contains all the vitamins other than A, D, E, and K. The water-soluble vitamins are excreted in the urine, compared with the fat-soluble vitamins, which are not. The other main difference between the two groups is the water-soluble vitamins are not stored in the body in any appreciable amount. Already, you have some useful information. By knowing that water-soluble vitamins are not stored in the body, we then realize that we must replenish these vitamins often.

Each vitamin is designated by its own letter. Denoting a particular vitamin by a letter makes it both easy to identify and of course, quicker to write. For example, ascorbic acid is given the letter "C". These designations are less cumbersome, yet still convey the vitamin. Let's construct a simple list some of the common vitamins that we may have heard of, and their letter designations.

vitamin A - this is a collective term for retinol and related compounds.

 

vitamin B complex - A group of more than a dozen water-soluble substances.

 

vitamin C - ascorbic acid

 

vitamin D - ergocalciferol

 

vitamin E - tocopherol and other related compounds with antioxidant activity.

 

vitamin K - phytonadione and others in this group of structurally related compounds which promote blood clotting

 

Now that we have learned that each vitamin has its own letter designation, we can discuss number designations. We learned above that ergocalciferol is known as vitamin D. There are a few different types of vitamin D. To keep things simple, we use a subscript (small number to the right and a little below the vitamin letter) to designate the different types. It's really quite simple. Vitamin D₂ is the designation for ergocalciferol. Vitamin D₃ is the designation for cholecalciferol. Now that you have learned all the designations used for vitamins, let us now construct a more complete list.

 

 

We can even further divide some vitamins into a smaller category by placing a letter next to the number.

Don't let this confuse you. By adding the letter "A" to the B₁₂ it simply means that this vitamin (B_12a) is similar in structure to B₁₂ ,but has a minor modification, making it slightly different. This is just like owning a VW Golf, E or EF. They are both VW Golfs, but one of them has a slight modification. It's that simple.

Just in case your extra enthusiastic and want to know the difference between these two forms of vitamin B₁₂, they only differ slightly in structure. Vitamin B_12a is similar to B₁₂, with the exception of an added hydroxyl group. A hydroxyl group is merely an oxygen and hydrogen. However, this difference in chemical structure enables them to perform slightly different functions in different areas of the body. See how easy this is. And the good news? We are going to teach all the different structures to you ! weeeeeeeeeeeeee !!!

 

 

We can obtain vitamins (as well as other nutrients) from 1)our diet 2) bacterial gut flora (small amounts, insufficient) and 3)exogenous sources (ie., pills, powders, liquids, intravenous, etc.,). Depending on the type of medium consumed, digestion and release of vitamins and other foodstuffs can either begin in the mouth or the stomach.

If a food is being consumed and is being chewed, digestion is beginning in the mouth. Obviously, grinding food with teeth breaks it down. To help speeden the process, saliva is liberated in the mouth. Saliva is a mixture of water, enzymes, and a few other substances. As the food is chewed, saliva mixes with the bolus of food. An enzyme present in the saliva called salivary amylase (also known as alpha-amylase), helps to start breaking the chemical bonds between complex carbohydrates (sugars) in the food (if carbohydrates are present), and thus, releases them as free branches, or free sugars. As food is broken down further from chewing, more of the complexes holding the vitamin are broken down. This helps to release vitamins for absorption, once the food source reaches the small intestine. Once the food is chewed enough and swallowed, the bolus enters the stomach. Carbohydrate digestion is temporarily halted by the stomach. The salivary amylase that was liberated in the mouth to start carbohydrate digestion is deactivated by the low pH (increased acidity) of the stomach. The stomach creates an acid environment by creating hydrochloric acid. The hydrochloric acid (HCL for convenience) in the stomach, breaks down the amylase, and also helps a bit in digestion.

 

Aided by its muscular walls, the stomach further mixes the bolus. The food is then squeezed from the stomach into the small intestine. Once the food is in the small intestine, enzymes are liberated form the pancreas to continue digestion. As the food is further broken down, the released vitamins can be absorbed through the barrier of the small intestine. Fats and fat-soluble vitamins are digested with the help of pancreatic enzymes and bile. Bile is a mixture created in hepatocytes (cells) of the liver, and stored in the gallbladder, a sac neatly tucked away, next to the liver. Once a fatty meal enters the small intestine, the gallbladder is stimulated to contract and squeeze out bile. The bile travels from the gallbladder to the small intestine, via a thin duct (tube), just in time to mix with the food present in the small intestine. Bile functions in mixing with fats (and fat soluble vitamins), increasing the surface area and thus helping them to be absorbed through the small intestinal wall at a quicker rate. Once vitamins and other nutrients pass through the walls of the small intestine, they are transported through the blood by transport proteins, other substances, and sometimes by themselves.

Every vitamin (and other nutrient) has its own story of absorption, function or functions, and excretion. We shall discuss each vitamin in detail in order to gain a complete picture, beginning at the nutrient source, and following it throughout the body until it is finally excreted.

 

 

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Vitamin A-A vitamin. Vitamin A is a collective (umbrella) term for three different molecules. The three molecules are named retinol, retinal, and retinoic acid. By examining the diagram below, we can see that these three different forms of vitamin A affect different tissues in the body.

Retinal is the aldehyde obtained from the oxidation of retinol. Basically, retinal and retinol are similar in structure and can be interconverted to each other, depending on need.

Retinol (also called vitamin A₁) is what we call a primary alcohol. Retinol can be found in animal tissues and exists in a structure combined with a fatty acid. We can see how this can contribute to retinol's fat-soluble nature, as discussed in our introduction to vitamins.

Retinoic acid is the acid product of oxidized retinal. Above we mentioned that retinol and retinal can be interconverted depending on need. Retinoic acid, in comparison, cannot be interconverted. Once retinoic acid is formed it remains in this form and is used up in maintenance of epithelial cells (skin cells lining cavities in body and on skin surface) Read about retinoic acid and its application to anti-wrinkle creams and it's risks.

Beta-carotene-One of four similar pigments, found in plants and having colours ranging from violet to yellow. Each of the four carotenes is preceded by a Greek letter (alpha, beta, gamma, or delta) and merely indicates the slight difference in structure. Carotenes are found in many dark green, leafy and yellow vegetables such as carrots, sweet potatoes, squash, and turnips. Carotenes are also found in yellow fruits such as oranges peaches, apricots, cantaloupes, etc.). They are fat-soluble, and can be split into 2 molecules of retinal by enzymes in the intestinal wall and the liver. In humans, B-carotene is the major provitamin (precursor) of vitamin A. B-carotene is not absorbed as well as retinol, but is in greater availability in the diet.

Beta-carotene can be used as a provitamin (precursor) of vitamin A since it can be split into 2 molecules of retinal. Oral Beta-carotene preparations have also been used to reduce the severity of photosensitivity in patients with certain types of porphyria. Beta-carotene has also been used as a method of artificial tanning.

As the food we've consumed is digested, retinyl esters are released. With the help of bile (which helps aid in the digestion of fats), these esters are absorbed into the intestinal mucosa and are hydrolyzed (broken apart), releasing retinol and free fatty acids (fats or lipids). Retinol from these esters and from carotenes as well, are re-esterified (bonded to a fat again) in the intestinal mucosa and secreted into the lymphatic system as chylomicrons. The lymphatic system is a network of vessels which travel parallel to blood vessels and carry proteins and fat. When retinyl esters in chylomicrons reach the liver, they are taken up and stored, until needed. When vitamin A is needed, retinol is released from the liver, and transported to the tissue in need. Retinol is transported by a protein called retinol-binding protein (RBP). There are specific receptors on RBP which are attracted to receptors on tissues that are in need of vitamin A. When receptors from the tissue and RBP interact, retinol is released from RBP, where it enters the tissue.

Retinoic acid (a derivative of retinol) is thought to be the form of vitamin A that plays a role in bone growth and development.

Vitamin A plays a vital role in the visual cycle. Vitamin A is part of the rhodopsin, the visual pigment of the rod cells in the retina. Rhodopsin consists of Delta¹¹-cis-retinal (simply a chemical description of the molecule) bonded to a protein called opsin. When a photon (an amount of light) of light penetrates the eye and hits the retina (back of the eye), the rhodopsin undergoes a photochemical (light + chemical) reaction and dissociation of all-trans-retinal and opsin. When this dissociation occurs, it stimulates a nervous impulse. This nervous impulse travels through the optic nerve to the brain, and we interpret this as vision. The by-products of these chemicals are sent back to the liver for processing and return to the retina. Vision is a very interesting chemical event.

Vitamin A derivatives retinal and retinol are needed for reproductive processes, such as spermatogenesis (sperm formation), and preventing fetal resorption in the female. Retinoic acid is not chemically able to act in this capacity. Animals given vitamin A in the form of retinoic acid from birth become blind and sterile.

The retinoic acid form of Vitamin A is needed for the functional integrity of epithelial cells. This integrity includes maintenance and mucous secretion.

Good sources of vitamin A are liver, kidney, egg yolk, butter, and cream. Yellow vegetables and fruits (discussed above) are good sources of the carotenes. Red palm oil is also a good source of Beta-carotene, which functions as provitamin A.

The recommended daily allowance (RDA) for an adult male is 1000 retinol equivalents (RE) and 800 RE for the female. A retinol equivalent is simply a quantity of retinol or another form of the vitamin. Remember, that vitamin A exists in several forms and we can obtain it from breaking down carotene. Therefore, 1 RE= 1 microgram of retinol. 1 RE also equals 6 micrograms of carotene, or 12 micrograms of other carotenoids. Remember what we discussed above about the absorption of retinol. Retinol is absorbed better than carotenes. However, we get more carotenes in the diet. Therefore we have to consume more carotenes to get the same effect as less retinol. Thus the change in retinol equivalents. Another measurement of vitamin A that is used is called the international unit (IU). 1 microgram of retinol=3.33 IU. Don't let this confuse you. The more you use it, the easier it gets. Just copy the chart below for reference and stick it on your refrigerator :o)

 

One of the earliest signs of vitamin A deficiency is night blindness, also called nyctalopia. Short term deficiency is usually reversible, but chronic vitamin A deficiency usually leads to irreversible loss of visual cells. Sever deficiency of vitamin A can lead to xerophthalmia, a dryness of the conjunctiva and cornea. This leads to ulceration and blindness if untreated. Xerophthalmia is usually only seen in third world countries. However, night blindness has a worldwide distribution. Deficiencies of vitamin A can result in less food intake. This may be due to loss of taste through keratinization of the taste buds. Decreased food intake will additionally affect growth. With decrease in bone growth, the skeletal system fails to keep parallel growth rate with the central nervous system. This results in central nervous system damage. As we can see, vitamin A is important all along the developmental phase of the child, and throughout adulthood.

Do not make the mistake of taking vitamin A during pregnancy ! Administration of vitamin A during pregnancy can result in malformation of the fetal brain. This is irreversible.

Signs and symptoms of toxicity of vitamin A have been recorded at daily doses of 25,000 IU. Symptoms of acute hypervitaminosis A include nausea, vomiting, headache, diarrhoea, stupor, and papilledoema, all suggestive of a brain tumour, interestingly enough. Chronic toxicity is associated with weight loss, nausea and vomiting, dryness of lips, bone and joint pain. Synthetic retinoids used in the treatment of acne do not give the same symptoms, but as with vitamin A, should be avoided in pregnancy, because of the risk of congenital malformations.